1. Field of the Invention
The present invention relates to a discharge lamp lighting device which lights discharge tubes while maintaining constant the respective tube currents of an arbitrary number of discharge tubes.
2. Description of the Related Art
A conventional discharge lamp lighting device which maintains tube current constant is disclosed in Japanese Utility Model Application Laid-Open No. 5-80191. In the discharge lamp lighting device disclosed in this publication, a tube current detected at a tube current detecting circuit is fed-back to a voltage control means provided so as to precede to a lighting circuit. The voltage control means is operated so that the tube current flowing through the discharge tube is constant, and lighting voltage is supplied to the lighting circuit. In the tube current detecting circuit, usually, the tube current flows through a resistor, and the circuit outputs the voltage generated across both ends of the resistor. Such a discharge lamp lighting device is generally called a current feedback type lighting device. In a liquid crystal backlight which uses a cold cathode discharge tube having an extremely small diameter and relatively high impedance as a light source, drawbacks arise in that the startability deteriorates and characteristics tend to fluctuate due to the effect of stray which occurs because a metal reflecting film is near a periphery of the discharge tube. These drawbacks tend not to arise in a current feedback type lighting device which provides a lit state in which the startability and characteristics are stable.
However, in the above-described conventional art, in a case in which the number of discharge tubes is varied arbitrarily, a current of a value which is the sum total of the currents flowing through the plurality of discharge tubes flows to the resistor of the tube current detecting circuit. Therefore, when an attempt is made to keep the voltage across both ends of the resistor constant, a drawback arises in that the current flowing per discharge tube varies. Further, if a source of current, a lighting circuit, a tube current detecting circuit or the like are provided respectively for each of the discharge tubes, the respective tube currents are held constant even if the number of discharge tubes is varied arbitrarily, but the lighting device becomes large, complex, and more expensive.